1. Field of the Invention
The present invention relates to a plastic lens elements supporting structure and more particularly to a plastic lens elements supporting structure which will not deteriorate due to temperature changes when applied to such a device as a focus detecting device for a camera in which a pair of lens elements are required to be compactly arranged and to keep their positional accuracy for a long time.
2. Description of the Prior Art
FIG. 2 shows a focus detection optical system of a known focus detecting device for a camera disclosed in, for example, U.S. Pat. Nos. 4,526,458 and 4,529,287. Referring to the drawing, 10 and 12 denote plastic image forming lens elements formed into an integral body, 2 denotes an objective lens, and 6 denotes a condenser lens. The condenser lens 6 and the elements on the right thereof are arranged into a single unit denoted by 8 in FIG. 1 to be mounted in a camera. From FIG. 1, it will be understood how strongly miniaturization of the focus detecting device is demanded and how advantageous it is to form the image forming lens elements of a plastic material into an integral body.
According to the known focus detecting device, a primary image 28 of an object is formed by the objective lens 2. The primary image is reimaged as a pair of secondary images 30 and 32 (or 34 and 36 or 38 and 40) by the image forming lens elements 10 and 12 and sensed by image sensing means (not shown in FIG. 12). The distance between the secondary images varies with the focus condition of the objective lens 2, i.e., the position of the primary image 28 relative to a predetermined focal plane 22 of the objective lens. The device detects the distance between the secondary images based on the output of the image sensing means, compares the detected distance with a reference distance which is the distance of the secondary images in an in-focus condition and then calculates the amount of defocus of the primary image 28, i.e., the amount of deviation of the primary image 28 from the predetermined focal plane 22, in acccordance with the comparison result, i.e., the difference between the detected distance and the reference distance.
By the way, the secondary images are formed in the relative position as indicated by 30 and 32 in FIG. 2 in the in-focus condition, as indicated by 34 and 36 in a front-focus condition, and as indicated by 38 and 40 in a rear-focus condition. Such a focus detection system will hereinafter be called the phase difference detection type for convenience. When the focus detection unit 8 made up of the condenser lens 6, the image-forming lens elements 10, 12, and a photosensor array 42 as the above mentioned image sensing means is to be incorporated into the body of a single-lens reflex camera as shown in FIG. 1 (for example, at the lower portion of a not shown mirror box), it will be advantageous if the image-forming lens elements 10 and 12 are formed of a plastic material into an integral body since the unit 8 can then be structured into a compact module. However, since plastics in general have coefficient of linear expansion 10.sup.3 to 10.sup.4 times as large as that of glass (in case of acrylic resin, for example, the value of the coefficient is 6.times.10.sup.-5 /.degree.C.), the effect of the changes in the temperature on the focus detection becomes too large to be neglected if the image-forming lens elements 10 and 12 are formed of a plastic material into an integral body. More specifically, change in ambient temperature causes change in the distance between the image-forming lens elements. In the case of a device of the dimensions as indicated in FIG. 3, for example, the distance between the lens elements will be increased by 2 .mu.m when the temperature rises by 30.degree. C. If this change in the distance is, for example, converted into the deviation amount of the image formed by the objective lens 2 from the predetermined focal plane 22, it will correcspond to the magnitude of the deviation amount as large as 68 .mu.m. A camera is generally used within a temperature range from -20.degree. C. to +40.degree. C. (and sometimes beyond this range) and, in the case of a single-lens reflex camera, focus detecting accuracy as high as .+-.50 .mu.m is generally demanded. For accurate focus detection, therefore it is necessary to compensate the effect of the change in the distance between the lens elements due to thermal expansion. A method for such compensation employing an electrical circuit arrangement has been proposed in Ishida et al U.S. patent application Ser. No. 731, 453 filed on May 6, 1985, and assigned to the same assignee.
In the invention of the above mentioned patent application, it was assumed that there was an invariable relationship between the distance between the image-forming lens elements 10 and 12 and the ambient temperature. However, it has become apparent that the relationship between the lens-to-lens distance and the ambient temperature is complex and unsteady in the case where a plastic plate which is formed of a plastic material integrally with the image-forming lens elements 10 and 12 is fixed to a base body with an adhesive. FIG. 5 is a graph, in which the amount of deviation of a focused object image from the predetermined focal position is represented in the ordinate against the schedule of changes in the temperature plotted on the abscissa. The change in the amount of deviation obtained by an actual measurement is indicated by the solid line whereas the change in the amount of deviation obtained by a simulation based on an assumption that the plate 9 is supported in a completely free state is indicated by the dotted line. The measurement was made with the plastic plate 9 integral with the lens elements 10 and 12 fixedly attached to the base body with the adhesive A at the opposite ends thereof as shown in FIG. 4. Changes in the temperature from 30.degree. C. to 50.degree. C. and then back to 30.degree. C. for one round produced residual deformation corresponding to 24 .mu.m in terms of the amount of deviation of the object image from the predetermined focal position. The measured values were deviated from the results of the simulation throughout the process of the temperature changes. The amount of deviation as indicated by the dotted line can be compensated for by the electrical compensation method of the above Ishida et al U.S. patent application. However, even with such a compensation, the difference between the solid line and the dotted line becomes the error in the focus detection. Meanwhile, an adhesive is advantageous as means for fixedly attaching the plate 9 to the base body 13 because it not only contributes to reduction of the number of mechanical parts such as screws but also improves workability in attaching the plate to the base body. However, in the case of the attaching method of FIG. 3 in which the adhesive A is applied to the opposite end portions of the plate 9, the adhesive inevitably gets into a space between the plate 9 and the base body 13 to thereby form an adhesive layer which serves to shift the position of the plate 9 and thus those of the lens elements 10 and 12 in the direction of the optical axis. Such a position shift accidentally changes the distance between the two images formed on the photosensor array 42, resulting in failure of accurate focus detection.